There have been increasing demands for high speed data communication links between computers and between components of computers. Wirelines are typically used in high speed communication links, such as buses which are widely used to electronically connect electronic devices. High speed buses are utilized within a digital processor system to connect various components of the system, such as connecting memory to a CPU or other processing units.
For high speed data communication, a “differential” type of signal communication system has been found to be particularly advantageous. A pair of differential signals is transmitted over a pair of wires. Each wire transmits the same signal, but with different polarities. Differential signals provide higher signal to noise ratios, and better overall performance in part because signal distortions are minimized. For example, IEEE 1394 Standard specifies a high speed serial bus that transmits and receives differential signals over point-to-point links. Twisted pair or twin-x shielded cables for differential signals have been used for wiring high speed networks.
FIG. 2 illustrates a typical differential signaling system. Data transmitting device 201 contains differential signal driver 203. The signal driver converts input digital symbols on line 205 into differential signals on a pair of wires 221. Data receiving device 211 contains differential signal receiver 213, which determines the digital signal output states (the digital symbols transmitted through the communication lines) from the differential signals received from input lines 217 and 219. Differential receiver 213 outputs the digital signal output states, which correspond to the input digital symbols (205), on line 215 for further processing by data receiving device 211.
FIG. 3 shows typical differential signals used by a typical differential receiver. Voltages VH (303) and VL (301) represent the high and low voltage rails (e.g., the extreme voltages received at the differential receiver after a number of consecutive 1's have been transmitted) at the differential receiver. Signals S+ (311) and S− (313) correspond to the signals on input lines 217 and 219. It is seen that signals S+ and S− contain essentially the same signal, but with different polarities. When the digital signal output state is 1, signal S+ is higher than signal S−; when the digital signal output state is 0, signal S+ is lower than signal S−. Thus, a typical differential receiver compares the signal levels of S+ and S− periodically to determine the digital signal output states (the digital symbols being transmitted).
When the digital symbol being transmitted changes from 1 to 0 and then back to 1, for example from time t0 to t1 and then to t2 in FIG. 3, the voltage swing of S+ is v (309). Similarly, signal S− reaches maximum swing at time t1. From time t0 to t2, signals S+ and S− cross each other to form data eye 315. Data eye 315 must be sufficiently large for a typical differential receiver to reliably determine the digital signal output state from comparing the signal levels of S+ and S− at time t1. A data eye is characterized by width δt(305) and height δV(307).
FIG. 3 shows that different signal transmission states, which indicate the characteristics of the differential signals on the transmission lines, may be associated with different sequences of transmitted digital symbols. For example, the signal transmission state at time to is associated with a changing sequence of digital symbols and a smaller data eye; and the signal transmission state at time t2 is associated with a larger data eye. A traditional differential signal receiver has a higher probability in correctly detecting a transmitted digital symbol for some signal transmission states while having a smaller probability in correctly detecting a transmitted digital symbol for some other signal transmission states. The performance of the transmission line is limited by the smaller probability associated with the signal transmission states with smaller data eyes.
Typically, a signal driver (e.g., driver 203 in FIG. 2) is designed to force rapid changes in the differential signals when the transmitted digital symbols are changed. A rapid change enables a differential signal to swing from one rail to another quickly in order to form a large data eye. However, a rapid change in the differential signal contains high frequency Fourier components. Whether or not such high frequency Fourier components can be reliably transmitted may be severely restricted by the signal transmission system when the data transfer rate is high. Both skin effect and dielectric loss cause frequency dependent attenuation. As the frequency increases the attenuation increases. Skin effect limits the current for high frequency signals to the near surface region of a transmission wire, which leads to a significant increase in the resistance of the wire, resulting in high signal attenuation. Further, dielectric loss of the printed circuit broad may further attenuate the high frequency components of the signal. Furthermore, noise (e.g., intersymbol interference, crosstalk, reflections due to connectors or printed circuit board vias, and others) degrades a communication link in a way that is proportional to the frequencies of the Fourier components.